Complex hereditary spastic paraplegia associated with episodic visual loss caused by aco2 variants

ABSTRACT Most patients with homozygous or compound heterozygous pathogenic ACO2 variants present with muscular hypotonia features, namely, infantile cerebellar-retinal degeneration.

Recently, two studies reported rare familial cases of ACO2 variants presenting as complex hereditary spastic paraplegia (HSP) with broad clinical spectra. Here, we report the case of a

20-year-old Japanese woman with complex HSP caused by compound heterozygous ACO2 variants, revealing a new phenotype of episodic visual loss during febrile illness. SIMILAR CONTENT BEING

VIEWED BY OTHERS HOMOZYGOUS _TFG_ GENE VARIANTS EXPANDING THE MUTATIONAL AND CLINICAL SPECTRUM OF HEREDITARY SPASTIC PARAPLEGIA 57 AND A REVIEW OF LITERATURE Article 25 March 2021 A JAPANESE

HEREDITARY SPASTIC PARAPLEGIA FAMILY WITH A RARE NONSYNONYMOUS VARIANT IN THE _SPAST_ GENE Article Open access 25 May 2021 A CLINICAL AND GENETIC STUDY OF SPG31 IN JAPAN Article 07 February

2022 The _ACO2_ gene on chromosome 22 encodes the aconitase 2 (ACO2) protein in the mitochondrial matrix; ACO2 catalyzes the stereospecific isomerization of citrate to isocitrate in the

tricarboxylic acid (TCA) cycle1. Pathogenic _ACO2_ variants were first reported in eight individuals from two Arab families, and they had infantile cerebellar-retinal degeneration (ICRD,

OMIM#614559)2. Subsequently, ~20 cases of pathogenic homozygous or compound heterozygous _ACO2_ variants have been reported, including mild cases such as isolated optic atrophy (optic

atrophy 9, OMIM#616289)2,3,4,5,6. Most patients initially present with muscular hypotonia, ataxia, seizures, progressive optic atrophy, retinal degeneration, and intellectual disabilities.

Decreased aconitase activity in fibroblastic or lymphoblastic cells suggests that impaired energy metabolism in the TCA cycle is a major cause of symptoms in patients with pathogenic _ACO2_

variants. Recently, cases from two families with pathogenic _ACO2_ variants represented by early- or late-onset spastic paraplegia with intellectual disability and broad clinical spectra

were reported4,5. Here, we describe pathogenic variants in the _ACO2_ gene presenting as complex hereditary spastic paraplegia (HSP) with a new phenotype of episodic visual loss after every

febrile infection and progressive optic atrophy. This is the third familial report and the first Asian patient with complex HSP caused by pathogenic _ACO2_ variants. The proband was born to

nonconsanguineous healthy parents at 38 weeks gestational age after unremarkable delivery. She did not have a family history of neuromuscular disorders or motor development delay. Her birth

weight was 2482 g, and her head circumference was 32 cm. Her motor development was delayed, and she could not walk independently at 1 year and 10 months because of progressive lower limb

spasticity. Physiotherapy was subsequently provided, and she started walking independently at 2 years and 6 months. Her cognitive level was moderate disability (estimated development

quotient: 50) at this time. From 3 years of age, she experienced recurrent encephalopathy-like episodes, episodic visual loss, ataxia, and altered consciousness after every febrile illness

episode. During febrile illness, she often accidentally hit her head on the wall because of her poor vision. Her visual loss recovered after defervescence, although the other symptoms

remained for several weeks. In the acute phase, magnetic resonance imaging (MRI) of the cerebrum and the retrobulbar optic nerve and ophthalmoscopy revealed no abnormalities. The following

laboratory results were normal: blood cell count; routine serum chemistry; glucose, ammonia, creatine kinase, lactate, and amino acid levels; and thyroid function. The urinary organic acid

and amino acid profiles and the cerebrospinal fluid (CSF) results for cells, glucose, protein, and lactate were within normal limits. The electroencephalogram showed diffuse slow waves and

focal spikes compatible with nonspecific encephalopathy; subsequently, antiepileptic drug therapy was initiated. However, despite treatment with the medications, episodic attacks repeatedly

occurred after every episode of fever. Her lower limb spasticity and reflexes progressed with sustained clonus and extensor plantar responses. At 18 years of age, she was admitted to our

hospital with acute psychomotor agitation after infection. Cerebral and spinal MRI, CSF analysis, metabolic screening, and ophthalmological evaluations were performed during admission.

Laboratory results showed no abnormalities, whereas cerebral MRI showed mild cerebellar vermis atrophy, and ophthalmoscopy showed bilaterally pale optic discs and suspected optic atrophy

(Fig. 1A, B). These findings suggested a genetic cause for the complex HSP. Written informed consent was obtained from her parents in accordance with the Review Board and Ethics Committee of

Kyoto University, and whole-exome sequencing (WES) was performed when she was 19 years old. Trio-based WES was performed using the SuperSelect XT Human All Exon v6 (Agilent Technologies,

Santa Clara, CA). Captured libraries were sequenced using NovaSeq 6000 (Illumina, San Diego, CA). WES identified compound heterozygous missense variants in _ACO2_. The first variant was in

exon 6 (NM_001098.2: c.776 G > A, p.Gly259Asp) and was predicted to be deleterious by SIFT (score 0; http://sift.jcvi.org/) and disease-causing by MutationTaster (prob 1;

http://www.mutationtaster.org/). This variant is known to be a disease-causing mutation: rs786204828 (pathogenic)6. The second variant was located in exon 17 (NM_001098.2: c.2148 C > G,

p.Asn716Lys) and has not been reported as a pathogenic variant; it was found to have an extremely low allele frequency (1.59 × 10−6) in the Genome Aggregation Database

(http://gnomad.broadinstitute.org). This variant was predicted to be deleterious by SIFT (score 0.04) and disease-causing by MutationTaster (prob 0.999). Both variants were confirmed by

Sanger sequencing; the p.Gly259Asp and p.Asn716Lys variants were found to be maternally and paternally inherited, respectively. The unaffected younger sister had a heterozygous p.Gly259Asp

variant inherited maternally (Fig. 1C). We evaluated the pathogenicity of these two variants in accordance with the 2015 guidelines of the American College of Medical Genetics and Genomics.

The c.776 G > A, p.Gly259Asp and c.2148 C > G, p.Asn716Lys variants were classified as pathogenic and likely pathogenic, respectively. At the age of 20 years, the proband had severe

cognitive function (estimated intelligence quotient: ~30) and moderate visual impairment. She walked on her toes with spastic scissor gait and required a walking aid. Mitochondrial ACO2 is a

critical enzyme in the TCA cycle, which is the primary source of cellular metabolic energy1. Other TCA enzymopathies, such as deficiencies of alpha-ketoglutarate dehydrogenase, fumarase,

succinate dehydrogenase, and succinyl-CoA synthase, have been previously reported to cause severe encephalopathy with muscle hypotonia, developmental delay, and retinitis

pigmentosa7,8,9,10,11. The underlying pathophysiological mechanism may involve a disruption of energy metabolism and oxidative phosphorylation7. Therefore, aconitase deficiency is also

thought to disrupt cellular energy metabolism; consistent with this, a study reported mitochondrial dysfunction in the fibroblasts of an aconitase-deficient patient12. Aconitase enzymopathy

is more difficult to diagnose than other TCA enzymopathies. One reason is the poor abnormalities in metabolic screening samples. An elevation of lactate levels in the blood and CSF and of

specific organic acids in the urine, which is typically detected in other TCA enzymopathies, is not observed in aconitase enzymopathy. Another reason is the broad clinical spectrum of

_ACO2-_related disorders, ranging from isolated optic atrophy to syndromic optic atrophy, such as ICRD that involves hypotonia, retinal degeneration, severe encephalopathy, epilepsy, and

cerebellar ataxia6. Moreover, new phenotypes associated with spastic paraplegia were recently reported, including in this study. Table 1 shows the clinical manifestations of _ACO2_-related

disorders (optic atrophy 9, ICRD, and complex HSP)4,5,6,13. Most patients with _ACO2_-related disorders have optic nerve involvement, which might be a hallmark feature of aconitase

enzymopathy, but not all patients have optic atrophy4,12. Thus, biochemical testing and clinical phenotypes are insufficient for diagnosing aconitase enzymopathy, thereby indicating the

importance of WES. Figure 1D shows the structure of the ACO2 protein and reported variants in _ACO2_-related disorders, including in this study. There is no hot-spot region in any

_ACO2_-related disorder, and there seems to be poor genotype–phenotype correlation. Compared to previously reported _ACO2_-related disorders, the novel characteristic phenotype in the

present patient was episodic visual loss during febrile infection (Table 1). The phenotypes in the present patient indicated that ACO2 plays a crucial role in energy production in the optic

nerve and retina, which are highly energy-dependent structures14. Previous findings suggested that phenotype variation and severity depend on residual aconitase enzymatic activity12.

Metodiev et al. reported a very severe case involving homozygous Gly259Asp variants6. The patient presented with syndromic optic neuropathy along with encephalopathy and cerebellar atrophy

and died at 57 days because of central apnea6. The aconitase enzymatic activity in the patient’s fibroblasts was extremely low (~5%)6. However, a case report of a mild phenotype despite a

marked reduction in aconitase enzyme activity12 suggested poor genotype–phenotype correlations in _ACO2_ variants and the coexistence of genomic modifiers. HSP is not a single disease;

rather, it is a mixture of genetically heterogeneous conditions resulting in broadly overlapping clinical phenotypes. Using single-gene direct sequencing and next-generation sequencing

technologies, various HSP-related gene variants have been identified15. These genes encode proteins with diverse molecular functions, axonal transport, specific lipid metabolism, synaptic

formation, axon development, and mitochondrial function15. In addition to _ACO2_ variants, several other HSP-associated gene variants, such as those in _PGN_, _HSPD1_, _DDHD1_, _REEP1_, and

_MT-ATP6_, have been found to impair mitochondrial function16,17,18,19,20. Further reports on the causative genes of HSP would improve the understanding of the crucial role of mitochondrial

dysfunction in HSP pathogenesis. In conclusion, this case represents the third report of HSP caused by pathogenic _ACO2_ variants. Although most patients with _ACO2_-related disorders

present with muscular hypotonia features, it should be recognized that pathogenic _ACO2_ variants comprise one of the causes of complex HSP. Patients with _ACO2_-related disorders should be

evaluated for signs such as early-onset spastic paraplegia, especially those with episodic visual loss after febrile infection and progressive optic atrophy. The identification of pathogenic

_ACO2_ variants in patients with HSP could contribute to the development of specific therapies against HSP caused by mitochondrial dysfunction. HGV DATABASE The relevant data from this Data

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(2011). Article  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS We thank the patient and her family members for their cooperation in this study. This work was supported by the

Japan Agency for Medical Research and Development (grant number JP19ek0109301). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Pediatrics, Graduate School of Medical Science,

Kyoto Prefectural University of Medicine, Kyoto, Japan Takenori Tozawa, Yoshihiro Taura & Tomohiro Chiyonobu * Department of Pediatrics, Ayabe City Hospital, Ayabe, Japan Takenori Tozawa

 & Tamaki Ueno * Department of Neonatology, Japanese Red Cross Society Kyoto Daiichi Hospital, Kyoto, Japan Akira Nishimura * Department of Pediatrics, Tokai Central Hospital,

Kakamigahara, Japan Tamaki Ueno * Kyoto Prefectural Maizuru Rehabilitation Center for Children, Maizuru, Japan Akane Shikata * Department of Pediatrics, Kyoto University Graduate School of

Medicine, Kyoto, Japan Takeshi Yoshida * Department of Medical Ethics/Medical Genetics, Kyoto University School of Public Health, Kyoto, Japan Naoko Nakagawa, Takahito Wada & Shinji

Kosugi * Center for Medical Genetics, Keio University School of Medicine, Tokyo, Japan Tomoko Uehara & Kenjiro Kosaki * Department of Pediatrics, Keio University School of Medicine,

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ARTICLE Tozawa, T., Nishimura, A., Ueno, T. _et al._ Complex hereditary spastic paraplegia associated with episodic visual loss caused by _ACO2_ variants. _Hum Genome Var_ 8, 4 (2021).

https://doi.org/10.1038/s41439-021-00136-y Download citation * Received: 13 November 2020 * Revised: 15 December 2020 * Accepted: 15 December 2020 * Published: 26 January 2021 * DOI:

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